Hot to make it when mob hits the player the player sets on fire

Is there not a trigger on the entity something like entity attacks other entity? If not use the global trigger: entity attacked. Either way use this same procedure:

If source entity is type chili creeper

Set event/target entity on fire for (10?) ticks

Create a procedure element in your workspace, unconnected to anything else, within the green block at the top select global trigger, then entity attacked.

Now check if source entity is chili creeper, and if yes, set target entity on fire for [number] seconds, just like fr6_17 said in their first comment.